1. Field of the Invention
The present invention relates to a process for producing trimellitic acid, which enables to be suitably used as a raw material in the fields of thermostable plasticizers and various heat-resistant polymers, by the oxidization of pseudocumene with a molecular oxygen containing gas in an acetic acid solvent in the presence of catalysts.
2. Description of the Prior Art
Trimellitic acid is a very useful raw material for thermostable plasticizers or various heat-resistant polymers (Thermoplastics), so it has been demanded to develop an economical and effective process which enables to obtain a pure trimellitic acid.
Various processes have been introduced previously, which produce trimellitic acid by oxidizing pseudocumene with a molecular oxygen containing gas in an acetic solvent in the presence of a catalyst comprising heavy metal compounds and one or more bromine compounds. The methods characterized by continuously conducting the oxidization are described in for instance, Japanese Patent Laid-open No. 57-128730, Japanese Patent Publication No. 58-39813, Japanese Patent Laid-open No. 57-167942, and U.S. Pat. No. 4,835,308.
However, each of the above methods is still not properly applicable in the practical use even though each method is advantageous to obtain a relatively good quality product continuously.
The method described in Japanese Patent Publication No. 58-39813 is not only a complicated process characterized by conducting the oxidization with multi-stage operations by changing the composition of the quarternary catalyst comprising cobalt, manganese, cerium and bromine from one stage to another, but also an uneconomical process because the recycling of the catalyst which is very expensive is practically impossible due to the necessity that the composition of the catalyst has to be adjusted in each stage.
The method described in Japanese Patent Laid-open No. 57-128730 is a multi-stage oxidization process characterized in that the reaction in the first stage of the batchwise manner is conducted by introducing a series of catalysts such as cobalt, manganese and bromine, in a state of relatively low temperature range (110.degree. C. -170.degree. C.) until the more than 2.8 mole oxygen per mole of pseudocumene is consumed and the oxidization is then completed in the second stage of the reaction which is a continuously-flow oxidization process with a constant high temperature. In said process, the reaction in the first stage is to start at constant temperature (example 1:110.degree. C.) and the temperature is to increase as the consumption rate of oxygen is gradually decreasing after passing through critical maximum value. The reaction is to be completed by means of supplying more than 2.8 mole oxygen per mole of pseudocumene after suspending elevating a temperature at the temperature (example 1:150.degree. C.) when oxygen is moderately actively consumed again.
This method, however, has disadvantages that the yield of and the selectivity for trimellitic acid are not sufficiently high since the oxidization of pseudocumene to carbon dioxide and water, which is the most undesired side reaction for the selective oxidization of pseudocumene to trimellitic acid occurs, and that the operational difficulty carefully to convert and control temperature in the first stage is severe.
The method described in Japanese Patent Laid-open No. 57-167942 is a continuous oxidization process, composed of two stage reactions that require two sets of consecutive reactors at each stage, is characterized in that the oxidization at the first stage of reaction is successively conducted by introducing cobalt, bromine and a little portion of manganese into the first reactor and the outflow from the first reactor is reacted again in the second reactor under the same condition as the first reactor until the concentration of the unreacted pseudocumene is less than 0.4% by weight. Thereafter, at the second stage of reaction, the rest of manganese thereof is added in the third reactor to oxidize the outflow from the second reactor and then the oxidization is completed in the fourth reactor.
However, this method is designed actually for improving the effectiveness of the second stage oxidization by the addition of manganese, but thereby the composition of the catalyst used in the first stage oxidization is not suitable and therefore the oxidization of pseudocumene to carbon dioxide and water, which is the most undesired side reaction for the selective oxidization of pseudocumene to trimellitic acid, is relatively remarkable. In addition, as the reaction proceeds, an increase of the amount of trimellitic acid may cause auto-controlling effect which diminish the reaction rate instead of auto-oxidization which increase it.
Therefore, the method described in Japanese Patent Laid-open No. 57-167942 has a series of disadvantages as follow: (a) the yield of and the selectivity for trimellitic acid are not sufficiently high, (b) the operation is complicated, and (c) the recycling of the used catalyst is practically difficult.
The method described in the U.S. Pat. No. 4,835,308 also relates to the two stage oxidization process characterized in that the first stage oxidization is conducted at 110.degree. C. -180.degree. C. by introducing the whole quantity of catalysts and the second stage oxidization at 180.degree. C. -230.degree. C. without any catalyst.
The above-mentioned method enables to solve the difficulty of the recycling of the used catalyst but the concentration of the pseudocumene in the initial oxidization is so extremely high that it can not control the rate of oxidization properly. Furthermore, it can provide an insufficient yield and an unsatisfactory purity of trimellitic acid because a low concentration of the dissolved oxygen in the solution cause to promote a radical dimerization reaction which enables to produce a by-product having a high boiling temperature.
Furthermore, as the reaction proceeds, the concentration of trimellitic acid increases. The increase thereof cause the formation of insoluble precipitations together with metallic catalyst, and as a result, it may considerably effect the contamination of trimellitic acid and decrease the yield of trimellitic acid.